One or more embodiments of the inventive subject matter relate to validating local capture of pacing pulses in multisite pacing delivery.
Implantable stimulation devices or cardiac pacemakers are a class of cardiac rhythm management devices that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart. As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any additional functions it may perform, such as cardioversion/defibrillation.
A pacemaker is comprised of two major components, a pulse generator and a lead. The pulse generator generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The lead, or leads, is implanted within the heart and has electrodes which electrically couple the pacemaker to the desired heart chamber(s). A lead may provide both unipolar and bipolar pacing and/or sensing configurations. In the unipolar configuration, the pacing pulses are applied and responses are sensed between an electrode carried by the lead and a case of the pulse generator or a coil electrode of another lead within the heart. In the bipolar configuration, the pacing pulses are applied and responses are sensed between a pair of electrodes carried by the same lead. Pacemakers are also described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (an atrium or a ventricle). A dual-chamber system stimulates and/or senses in at least one atrial chamber and at least one ventricular chamber. Recently, there has been the introduction of pacing systems that stimulate multiple sites in the same chamber, termed multisite stimulation systems.
When the patient's own intrinsic rhythm fails, pacemakers can deliver pacing pulses to a heart chamber to induce a depolarization of that chamber, which is followed by a mechanical contraction of that chamber. Pacemakers further include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial depolarizations (detectable as P waves) and intrinsic ventricular depolarizations (detectable as R waves). By monitoring cardiac activity, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
In order for a pacemaker to control the heart rate in the manner described above, the pacing pulses delivered by the device must achieve “local capture,” which refers to causing sufficient depolarization of the myocardium surrounding a particular pacing electrode such that a desired propagating wave of excitation and contraction result (i.e., a heartbeat). A pacing pulse that does not capture the heart is thus an ineffective pulse. An ineffective pulse not only wastes energy from the pacemaker, which has limited energy resources (e.g., a battery), but can have deleterious physiological effects as well. A number of factors can determine whether a given pacing pulse will achieve local capture, but the principal factor is the energy of the pulse. The pacing pulse energy (e.g., pacing output) is a function of current, voltage, and duration. The pacing output can be adjusted to modify pulse parameters such as pulse amplitude, pulse width, and/or pacing delay to achieve local capture along the corresponding pacing vectors in response to the adjusted pacing pulses.
The minimum pacing output necessary to achieve local capture by a particular pacing channel (e.g., pacing vector) is referred to as the “local capture threshold.” The local capture threshold is not fixed, but rather, may increase and decrease during of the course of a single day, on a daily basis, and/or in response to changes in cardiac disease status. To reduce current drain on the limited power supply, it is desirable to automatically adjust the pacemaker such that the amount of stimulation energy delivered to the myocardium is maintained at a level that will reliably capture the heart without wasting power. Such a process is often referred to as “automatic capture verification and threshold search” or “autocapture,” but may be referred to by other names.
Recent studies have suggested that bi-ventricular (BiV) pacing (e.g., delivering pacing stimulus between the left ventricle and the right ventricle) from at least two left ventricular (LV) sites can improve clinical outcome in patients undergoing cardiac resynchronization therapy (CRT). The improved clinical outcome is likely due to improved hemodynamic response that can be achieved using multi-site LV pacing, in comparison with conventional single site BiV pacing. Pacing at more than one site within the LV chamber is referred to as multisite left ventricular (MSLV) pacing. To provide such MSLV pacing, leads have been developed that include multiple electrodes for placement in the LV chamber. For example, St. Jude Medical Inc. (headquartered in St. Paul, Minn.) has developed the Quartet™ left ventricular pacing lead, which includes four pacing electrodes on the left ventricular lead—enabling up to 10 pacing configurations (e.g., pacing vectors). One goal of MSLV pacing is to cause local capture at each LV pacing site (e.g., along each pacing vector), so the resulting depolarization waveforms propagate throughout the LV chamber in a desired manner. Another goal of MSLV pacing is to deliver pacing pulses that have pacing energy outputs that slightly exceed the local capture thresholds to conserve the limited energy available from the battery of the pacemaker.
One challenge associated with MSLV pacing relates to detecting achievement of local capture and the failure to achieve local capture (e.g., lack or loss of local capture) along each pacing vector when multiple sites are being paced within the LV chamber. Local capture thresholds vary with different pacing vectors and locations. Therefore, as expected, local capture thresholds vary more dramatically with quadripolar LV leads (e.g., such as the Quartet™ lead) than bipolar or unipolar LV pacing leads. Local capture thresholds for each pacing vector also vary on an individual patient-by-patient basis, and even fluctuate within a single patient, as mentioned above. Therefore, each of the selected pacing vectors may have a different local capture threshold. One pacing pulse delivered at a given pacing output may achieve local capture if delivered along a first vector, but not if delivered along a second vector. There is a need to distinguish between local capture along one pacing vector and local capture along another pacing vector.
To detect whether a pacing pulse achieved local capture in surrounding tissue along a pacing vector, a sensing vector is used to monitor for an “evoked response” associated with a corresponding pacing vector following the pacing pulse. The evoked response is a depolarization waveform that results from the pacing pulse and evidences contraction of the paced chamber generally along the pacing vector. Another challenge associated with MSLV pacing relates to accurately monitoring evoked responses specific to individual pacing vectors, especially when the pacing pulses are delivered simultaneously or in close succession. For example, pacing at one LV site can result in pacing artifacts at a second LV site, which interferes with the sensed evoked response at the second LV site.
Although capture verification may be performed in a clinical setting by a clinician, the pacemaker itself may verify capture using an autocapture algorithm that is configured to detect lack of local capture and adjust pacing parameters automatically. Autocapture algorithms have been developed, but such algorithms have typically been designed for only one or two pacing pulses being delivered in a same cardiac chamber per cardiac cycle. Accordingly, such algorithms may not effectively control more than two pacing pulses being delivered within the same cardiac chamber per cardiac cycle. In addition to the challenges already discussed, it is also noted that providing additional pacing pulses (e.g., three or more pulses) per cardiac cycle increases the drain on the battery. Accordingly, it is important to improve energy efficiency such that the pacing pulses are delivered with respective pacing outputs that reliably achieve local capture but do not waste the limited energy available.